Formal [3 + 2] Cycloaddition of α-Imino Esters with Azo Compounds: Facile Construction of Pentasubstituted 1,2,4-Triazoline Skeletons

1,2,4-Triazole and 1,2,4-triazoline are important components of bioactive molecules and catalysts employed in organic synthesis. Therefore, the efficient synthesis of these components has received significant research attention. However, studies on their structural diversity remain lacking. Previously, we developed chiral phase-transfer-catalyzed asymmetric reactions of α-imino carbonyl compounds with α,β-unsaturated carbonyl compounds and haloalkanes. In this study, we demonstrate the formal [3 + 2] cycloaddition reaction of α-imino esters with azo compounds under Brønsted base catalysis, resulting in the corresponding 1,2,4-triazolines in high yields. The results revealed that a wide range of substrates and reactants can be applied, irrespective of their steric and electronic characteristics. The present reaction made the general preparation of 3-aryl pentasubstituted 1,2,4-triazolines possible for the first time. Furthermore, a mechanistic study suggested that the reaction proceeds without isomerization into the aldimine form.


Reaction Condition Optimization
The reaction conditions for the synthesis of 1,2,4-triazoline 3aa were optimized using α-imino ester 1a and diisopropyl azodicarboxylate (DIAD, 2a) as the substrate and reactant, respectively (Table 1). Solvent screening was conducted using 1.0 equivalent of 1a and 2.0 equivalent of 2a in the presence of 50 mol% 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) as a base at −40 • C for 18 h. The reaction in nonpolar solvents, such as toluene, provided 3aa in only a 9% yield, and the use of ethereal solvents and methanol resulted in poor yields (entries [1][2][3][4][5]. Finally, the reaction in dichloromethane afforded 3aa in a 43% yield. Next, the amount of DBU was screened, and the use of 100 and 150 mol% DBU did not increase the yield of 3aa (entries 6 and 7). Subsequently, the effect of the reaction temperature was examined at −20 • C, 0 • C, and room temperature, and the reaction at 0 • C produced 3aa in the highest yield of 61% (entries 8-10). The reaction was completed after 1 h (entries 11 and 12). The reaction workup procedure was changed from short column on silica gel to extraction with dichloromethane, which increased the yield of 3aa to 88%, and 3aa was isolated in a 72% yield (entry 13). Finally, triethylamine was employed as an inexpensive organic base; however, 3aa was obtained in a low yield (entry 14).

Reaction Condition Optimization
The reaction conditions for the synthesis of 1,2,4-triazoline 3aa were optimized using α-imino ester 1a and diisopropyl azodicarboxylate (DIAD, 2a) as the substrate and reactant, respectively (Table 1). Solvent screening was conducted using 1.0 equivalent of 1a and 2.0 equivalent of 2a in the presence of 50 mol% 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) as a base at −40 °C for 18 h. The reaction in nonpolar solvents, such as toluene, provided 3aa in only a 9% yield, and the use of ethereal solvents and methanol resulted in poor yields (entries [1][2][3][4][5]. Finally, the reaction in dichloromethane afforded 3aa in a 43% yield. Next, the amount of DBU was screened, and the use of 100 and 150 mol% DBU did not increase the yield of 3aa (entries 6 and 7). Subsequently, the effect of the reaction temperature was examined at −20 °C, 0 °C, and room temperature, and the reaction at 0 °C produced 3aa in the highest yield of 61% (entries 8-10). The reaction was completed after 1 h (entries 11 and 12). The reaction workup procedure was changed from short column on silica gel to extraction with dichloromethane, which increased the yield of 3aa to 88%, and 3aa was isolated in a 72% yield (entry 13). Finally, triethylamine was employed as an inexpensive organic base; however, 3aa was obtained in a low yield (entry 14). Furthermore, the scope of the azo compounds was investigated using 1a as a substrate under the optimal conditions (Scheme 2). The use of DIAD formed 3aa in a 72% yield, and the utilization of diethyl azodicarboxylate (DEAD, 2b) or di-tert-butyl azodicarboxylate (2c) resulted in corresponding products 3ab and 3ac in 53% or 78% yields, respectively. These observations indicated that increasing the bulkiness of both the substrate and reactant increases the yield of the product. The employment of azobenzene (2d) did not provide any cyclized product 3ad. Scheme 2. Scope of azo compounds.
Subsequently, the substrate scope of the R 1 group was determined. The substrate scope using the inexpensive 2a and 2c as the reactants is presented in Scheme 3. In the case of 2a as a reactant, R 1 groups with electron-donating substituents, such as p-tolyl and p-anisyl groups, were examined, and the products 3fa and 3ga were isolated in 61% and 65% yields, respectively. Substrates with m-and o-tolyl groups were well tolerated, and 3ea and 3da were obtained in 53% and 60% yields, respectively. Furthermore, the electronwithdrawing substituents 1h, 1i, and 1j were used in the 1,2,4-triazoline synthesis, and the products were obtained in 43%, 50%, and 42% yields, respectively. The present reaction was successfully applied to several substituted substrates, and the products were obtained in moderate yields. Further substrate scope studies were conducted using the bulky azo compound 2c as the reactant. First, the same substrates used for evaluating the substrate scope using DIAD (2a) were employed. The products 3dc-3jc were obtained in 65-87% yields, which were higher than those obtained using DIAD as the reactant. Moreover, 2-naphthyl-substituted 1k and tert-butyl-substituted 1l were applied to the present reaction, which formed 3kc and 3lc in 76% and 29% yields, respectively. These Furthermore, the scope of the azo compounds was investigated using 1a as a substrate under the optimal conditions (Scheme 2). The use of DIAD formed 3aa in a 72% yield, and the utilization of diethyl azodicarboxylate (DEAD, 2b) or di-tert-butyl azodicarboxylate (2c) resulted in corresponding products 3ab and 3ac in 53% or 78% yields, respectively. These observations indicated that increasing the bulkiness of both the substrate and reactant increases the yield of the product. The employment of azobenzene (2d) did not provide any cyclized product 3ad. Furthermore, the scope of the azo compounds was investigated using 1a as a substrate under the optimal conditions (Scheme 2). The use of DIAD formed 3aa in a 72% yield, and the utilization of diethyl azodicarboxylate (DEAD, 2b) or di-tert-butyl azodicarboxylate (2c) resulted in corresponding products 3ab and 3ac in 53% or 78% yields, respectively. These observations indicated that increasing the bulkiness of both the substrate and reactant increases the yield of the product. The employment of azobenzene (2d) did not provide any cyclized product 3ad.

Scheme 2. Scope of azo compounds.
Subsequently, the substrate scope of the R 1 group was determined. The substrate scope using the inexpensive 2a and 2c as the reactants is presented in Scheme 3. In the case of 2a as a reactant, R 1 groups with electron-donating substituents, such as p-tolyl and p-anisyl groups, were examined, and the products 3fa and 3ga were isolated in 61% and 65% yields, respectively. Substrates with m-and o-tolyl groups were well tolerated, and 3ea and 3da were obtained in 53% and 60% yields, respectively. Furthermore, the electronwithdrawing substituents 1h, 1i, and 1j were used in the 1,2,4-triazoline synthesis, and the products were obtained in 43%, 50%, and 42% yields, respectively. The present reaction was successfully applied to several substituted substrates, and the products were obtained in moderate yields. Further substrate scope studies were conducted using the bulky azo compound 2c as the reactant. First, the same substrates used for evaluating the substrate scope using DIAD (2a) were employed. The products 3dc-3jc were obtained in 65-87% yields, which were higher than those obtained using DIAD as the reactant. Moreover, 2-naphthyl-substituted 1k and tert-butyl-substituted 1l were applied to the present reaction, which formed 3kc and 3lc in 76% and 29% yields, respectively. These Subsequently, the substrate scope of the R 1 group was determined. The substrate scope using the inexpensive 2a and 2c as the reactants is presented in Scheme 3. In the case of 2a as a reactant, R 1 groups with electron-donating substituents, such as p-tolyl and p-anisyl groups, were examined, and the products 3fa and 3ga were isolated in 61% and 65% yields, respectively. Substrates with m-and o-tolyl groups were well tolerated, and 3ea and 3da were obtained in 53% and 60% yields, respectively. Furthermore, the electronwithdrawing substituents 1h, 1i, and 1j were used in the 1,2,4-triazoline synthesis, and the products were obtained in 43%, 50%, and 42% yields, respectively. The present reaction was successfully applied to several substituted substrates, and the products were obtained in moderate yields. Further substrate scope studies were conducted using the bulky azo compound 2c as the reactant. First, the same substrates used for evaluating the substrate scope using DIAD (2a) were employed. The products 3dc-3jc were obtained in 65-87% yields, which were higher than those obtained using DIAD as the reactant. Moreover, 2-naphthyl-substituted 1k and tert-butyl-substituted 1l were applied to the present reaction, which formed 3kc and 3lc in 76% and 29% yields, respectively. These results show that the present reaction is applicable to both aryl-and alkyl-substituted substrates. results show that the present reaction is applicable to both aryl-and alkyl-substituted substrates. Scheme 3. Substrate scope using 2a and 2c as reactants.
Next, we examined the necessity for a 4-nitrobenzyl moiety on the substrate (Scheme 4). 4-Trifluoromethylbenzyl-substituted 1m and benzyl-substituted 1n were prepared and applied to the present reaction, which did not afford any 1,2,4-triazoline products. Only the substrate and its hydrolysis product were obtained together with the complex mixture, thereby indicating the importance of the 4-nitro group on the benzyl moiety in the production of 1,2,4-triazolines. Next, we examined the necessity for a 4-nitrobenzyl moiety on the substrate (Scheme 4). 4-Trifluoromethylbenzyl-substituted 1m and benzyl-substituted 1n were prepared and applied to the present reaction, which did not afford any 1,2,4-triazoline products. Only the substrate and its hydrolysis product were obtained together with the complex mixture, thereby indicating the importance of the 4-nitro group on the benzyl moiety in the production of 1,2,4-triazolines.

Asymmetric Synthesis
The asymmetric synthesis of 1,2,4-triazolines was attempted to demonstrate the utility of this reaction (Scheme 5). Here, 1a was reacted with 2c in the presence of 2.0 mol% of chiral phase-transfer catalyst 4 and 150 mol% of potassium hydroxide in dichloromethane at 0 °C, which provided 1,4-addition product 5ac in a 57% yield together with a small amount of the desired 3ac. Notably, 5ac was converted into 1,2,4-triazole 3ac using a 1.0 equivalent of 2c and 50 mol% of DBU in dichloromethane in a 47% yield. The enantiopurity of the synthesized 3ac was evaluated via high-performance liquid chromatography using a chiral stationary phase column, and it was found to be a racemate.

Reaction Mechanistic Study
Finally, to clarify the reaction pathway, α-imino ester 1a was isomerized into aldimine 1a′ because the α-imino ester isomerizes into aldimine under basic conditions [46]. Here, 1a′ was employed as the substrate under the same conditions as that of the asymmetric synthesis of 3ac, which directly provided 1,2,4-triazoline 3ac in a 65% yield Scheme 4. Scope of the aromatic substituent on the imine moiety.

Asymmetric Synthesis
The asymmetric synthesis of 1,2,4-triazolines was attempted to demonstrate the utility of this reaction (Scheme 5). Here, 1a was reacted with 2c in the presence of 2.0 mol% of chiral phase-transfer catalyst 4 and 150 mol% of potassium hydroxide in dichloromethane at 0 • C, which provided 1,4-addition product 5ac in a 57% yield together with a small amount of the desired 3ac. Notably, 5ac was converted into 1,2,4-triazole 3ac using a 1.0 equivalent of 2c and 50 mol% of DBU in dichloromethane in a 47% yield. The enantiopurity of the synthesized 3ac was evaluated via high-performance liquid chromatography using a chiral stationary phase column, and it was found to be a racemate.

Asymmetric Synthesis
The asymmetric synthesis of 1,2,4-triazolines was attempted to demonstrate the utility of this reaction (Scheme 5). Here, 1a was reacted with 2c in the presence of 2.0 mol% of chiral phase-transfer catalyst 4 and 150 mol% of potassium hydroxide in dichloromethane at 0 °C, which provided 1,4-addition product 5ac in a 57% yield together with a small amount of the desired 3ac. Notably, 5ac was converted into 1,2,4-triazole 3ac using a 1.0 equivalent of 2c and 50 mol% of DBU in dichloromethane in a 47% yield. The enantiopurity of the synthesized 3ac was evaluated via high-performance liquid chromatography using a chiral stationary phase column, and it was found to be a racemate.

Reaction Mechanistic Study
Finally, to clarify the reaction pathway, α-imino ester 1a was isomerized into aldimine 1a′ because the α-imino ester isomerizes into aldimine under basic conditions [46]. Here, 1a′ was employed as the substrate under the same conditions as that of the asymmetric synthesis of 3ac, which directly provided 1,2,4-triazoline 3ac in a 65% yield Scheme 5. Attempt at the asymmetric synthesis of 3ac.

Reaction Mechanistic Study
Finally, to clarify the reaction pathway, α-imino ester 1a was isomerized into aldimine 1a because the α-imino ester isomerizes into aldimine under basic conditions [46]. Here, 1a was employed as the substrate under the same conditions as that of the asymmetric synthesis of 3ac, which directly provided 1,2,4-triazoline 3ac in a 65% yield and a shorter reaction time; however, 5ac was not produced (Scheme 6). These results indicate that the reaction mechanisms for each substrate were different. and a shorter reaction time; however, 5ac was not produced (Scheme 6). These results indicate that the reaction mechanisms for each substrate were different. Scheme 6. Reaction from aldimine 1a′.
Based on the above results, we propose a plausible reaction mechanism ( Figure 2). First, the benzylic proton of substrate 1a is deprotonated by potassium hydroxide and its counteranion is changed to the chiral ammonium salt to form a 2-aza allyl anion intermediate, which attacks the azo compounds in a 1,4-addition reaction to yield ketimine 5ac-ionic. Finally, the cyclization of the hydrazine moiety with the imine moiety occurs, forming 1,2,4-triazolidine 6ac-ionic, followed by the oxidation of the amine part by the additional azo compound to afford 3ac [47]. In contrast, 1a′ reacts with potassium hydroxide to form an enolate intermediate, which is different from the reaction starting from 1a. The as-formed intermediate then reacts with azo compound 2c to form aldimine 5ac′-ionic, which undergoes cyclization to form 1,2,4-triazolidine 6ac. The reaction rate difference between 1a and 1a′ can be explained by these plausible reaction mechanisms. In this reaction, 1a did not isomerize into 1a′ under the reaction conditions, and the aldimine intermediate 5ac′-ionic could be cyclized more rapidly than the ketimine intermediate 5ac-ionic owing to its low steric hindrance around the electrophilic site. Therefore, the overall rate for the formation of 3ac increased, and no intermediate 5ac′ionic was observed, even after stirring for 18 h. Based on the above results, we propose a plausible reaction mechanism ( Figure 2). First, the benzylic proton of substrate 1a is deprotonated by potassium hydroxide and its counteranion is changed to the chiral ammonium salt to form a 2-aza allyl anion intermediate, which attacks the azo compounds in a 1,4-addition reaction to yield ketimine 5ac-ionic. Finally, the cyclization of the hydrazine moiety with the imine moiety occurs, forming 1,2,4triazolidine 6ac-ionic, followed by the oxidation of the amine part by the additional azo compound to afford 3ac [47]. In contrast, 1a reacts with potassium hydroxide to form an enolate intermediate, which is different from the reaction starting from 1a. The as-formed intermediate then reacts with azo compound 2c to form aldimine 5ac -ionic, which undergoes cyclization to form 1,2,4-triazolidine 6ac. The reaction rate difference between 1a and 1a can be explained by these plausible reaction mechanisms. In this reaction, 1a did not isomerize into 1a under the reaction conditions, and the aldimine intermediate 5ac -ionic could be cyclized more rapidly than the ketimine intermediate 5ac-ionic owing to its low steric hindrance around the electrophilic site. Therefore, the overall rate for the formation of 3ac increased, and no intermediate 5ac -ionic was observed, even after stirring for 18 h.  1 H-and 13 C-NMR spectra were recorded with Bruker (Billerica, MA, USA) AVANCE III-400M ( 1 H-NMR 400 MHz, 13 C-NMR 100 MHz, and 19 F-NMR 376 MHz). 1 H-NMR spectra are reported as follows: chemical shift in ppm (δ) relative to the chemical shift of CHCl3 at 7.26 ppm or tetramethylsilane at 0 ppm, integration, multiplicities (s = singlet, d  1 H-and 13 C-NMR spectra were recorded with Bruker (Billerica, MA, USA) AVANCE III-400M ( 1 H-NMR 400 MHz, 13 C-NMR 100 MHz, and 19 F-NMR 376 MHz). 1 H-NMR spectra are reported as follows: chemical shift in ppm (δ) relative to the chemical shift of CHCl 3 at 7.26 ppm or tetramethylsilane at 0 ppm, integration, multiplicities (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), and coupling constants (Hz). 13 C-NMR spectra are reported in ppm (δ) relative to the central line of triplet for CDCl 3 at 77 ppm. CF 3 CO 2 H was used as an external standard for 19 F. ESI-MS spectra were obtained with Thermo Fisher, Exactive (Waltham, MA, USA). FT-IR spectra were recorded on a JASCO FT-IR system (FT/IR-4X). HPLC analyses were performed on a JASCO HPLC system (JASCO PU 980 pump and UV-975 UV/Vis detector, Halifax, NS, Canada). Mp was measured with AS ONE ATM-02. Column chromatography on SiO 2 and neutral SiO 2 was performed with Kanto Silica Gel 60 (40-50 µm). All reactions were carried out under Ar atmosphere unless otherwise noted. Commercially available organic and inorganic compounds were purchased from TCI (Tokyo, Japan), Kanto Chemical Co. Inc. (Tokyo, Japan), Wako Pure Chemical Industries, Ltd. (Osaka, Japan), or Nacalai Tesque, Inc. (Kyoto, Japan), which had >95% purities, and were used without further purification. All dehydrated solvents were purchased from Wako Pure Chemical Industries, Ltd. or Nacalai Tesque, Inc., and were used without further purification.

Synthesis of Substrates and a Catalyst
Imine substrates 1 and 1a were synthesized according to the reported procedures [41,42,45]. Azo compounds were purchased from a commercial source. Chiral catalyst 4 was synthesized according to the reported procedure [42]. Table 1 A solution of 1a (1.0 equiv) in an appropriate solvent (0.05 M) was stirred for 10 min at the reaction temperature, and 2a (2.0 equiv) was added followed by DBU (appropriate amount). The reaction was stirred for an appropriate time at the same temperature before stopping the reaction. For the short-column procedure, the reaction mixture was directly passed through the short column (SiO 2 , ethyl acetate only) and evaporated to give the crude mixture. The NMR yield was determined by measuring its 1 H-NMR after adding 1,3,5trimethoxybenzene as an internal standard. For the extraction procedure, the reaction was quenched by the addition of excess amount of sat. NH 4 Cl aq. at the reaction temperature, which was extracted with CH 2 Cl 2 , dried over Na 2 SO 4 , and filtered. After the removal of solvent by evaporation, the crude product was obtained. The NMR yield was determined by measuring its 1 H-NMR after adding 1,3,5-trimethoxybenzene as an internal standard. 3aa was isolated through the purification by column chromatography (neutral silica gel, hexane/dichloromethane/diethylether = 7/2/1).

General Procedure for Schemes 1-4 (Optimized Protocol)
A solution of 1 (1.0 equiv) in CH 2 Cl 2 (0.05 M) was stirred for 10 min at 0 • C, to which 2 (2.0 equiv) was added, followed by DBU (50 mol%). The reaction was stirred for 1 h at 0 • C before quenching the reaction. The reaction was quenched by the addition of an excess amount of sat. NH 4 Cl aq. at the reaction temperature, which was extracted with CH 2 Cl 2 , dried over Na 2 SO 4 , and filtered. After the removal of solvent by evaporation, the crude product was obtained. The pure 3 was isolated through purification by column chromatography (neutral silica gel, hexane/dichloromethane/diethylether = 7/2/1).